Graphene is single planar carbon material having honeycomb crystal lattice the structure of which is formed of carbons by sp2hybridization bonding and was discovered most recently among carbon nanostructures. The shape of graphene is the same as that of a single layer derived by fully severance between layers from graphite which is stacked layers of carbon honeycomb crystal lattice.
In 2004, professor Andre Geim and et al from University of Manchester isolated graphene by peeling it off from graphite with Scotch tape and found excellent electric conductivity of the graphene so acquired by study of quantum hole effect. Thereafter, in 2008, James Hone and et al, researchers from university of Colombia confirmed the superior strength of graphene. In the same year, Alexander Balandin and et al, researchers from University of Riverside, Calif., measured the thermal conductivity of graphene as 5300 pW/mpK, which is double that of carbon nano-tube.
Longitudinal scission of carbon nanotube makes graphene structure and infinite enlargement of wall in carbon nanotube of single wall makes it similar to graphene. Therefore, the electronic, thermal and mechanical properties of graphene are expected to be comparable to those of carbon nano-tubes.
For preparation of graphene, drawing method by mechanical exfoliation (repeated peeling) of graphite crystals by the above-mentioned Andre Geim, epitaxial growth on substrates, hydrazine reduction on graphitic oxide paper, chemical vapor deposition and cutting open of nanotubes in solution of potassium permanganate and sulfuric acid have been known but none of them go beyond laboratory preparation levels.
On the other hand, the method of producing expanded graphite, the shape of which is worm-like or accordion-like, by adding thermal shock on graphite intercalated with acids or so on has been known since long before. Such worm-like expanded graphite is used as fillers or is rolled into sheets having anisotropic conductivity. Such expanded graphite resulting from loose bonding between layers of graphite is inferior to graphene in physical properties and its particulate size is much bigger than graphene.
Staudenmaier method wherein graphite flake is reacted with nitric acid and potassium perchlorinate for days has been known for preparing graphitic oxide. In addition, Hummers, in U.S. Pat. No. 2,798,878, shortened the reaction time by using sulfuric acid, nitric acid and potassium permanganate as oxidants. The mixing of sulfuric acid, nitric acid and potassium permanganate together is exothermic reaction and may cause explosion at around 70° C. or above. In these methods, Graphitic oxide is prepared by batch process in very small amount on each batch.